Platform:
Successfully tested on Linksys WAP54Gv3 loaded with firmware version
Ver.3.05.03 (Europe)
Vulnerability present also on firmware ver.3.04.03 (US)
Other models and/or firmware versions may be also affected.

Summary:
A cross-site scripting vulnerability is present in the debug.cgi page,
that is accessible by using proper debug credentials

Details:
The debug.cgi page act as debug interface for the Linksys WAP54Gv3 and
is accessible by authenticating with proper debug credentials at the
following URL:

http://AP_IP_ADDR/debug.cgi

where AP_IP_ADDR is the IP address of the device.

Commands to be executed by the system are sent within the data1 POST
variable, while the command output is returned within a <textarea> tag
in the output html page.
Output is not sanitized in any way, allowing for a Cross-site scripting
condition that can be triggered by any command that includes a
</textarea> closing tag in its output.
Additional text following such tag will be interpreted as regular HTML
by the accessing user browser, allowing for injection of Javascript
code, that will be run in the context of the presented web page.

Proof of Concept:
echo "</textarea><script>alert('XSS');</script>"

Impacts:
The vulnerability may allow an attacker to access the output of commands
during a "Remote blind" attack, where malicious web pages are used by
the attacker over the Internet to execute code on a victim access point
with private addressing, by leveraging an user browser as a 3rd party
"reflector".
This would also allow an attacker to extract information and
configuration stored on devices that are not even able to access the
Internet (eg: firewall policy, gateway not configured)

Solutions & Workaround:
Not available

Additional Information
----------------------
Timeline (dd/mm/yy):
09/11/2009: Requested Point of Contact to Linksys
10/11/2009: Received Point of Contact
10/11/2009: Vulnerability details sent
12/11/2009: Received clarification request on firmware version
12/11/2009: Additional details sent
16/01/2010: Requested update on vulnerability status.
----------- No update received -----------
23/06/2010: This advisory